Various standards exist with respect to supplying power around the world. For example, in the United States the power supply standard requires a power supply signal of 110 Volts alternating current at a frequency of 60 Hz and in Europe the power supply standard requires a power supply signal of 220 Volts alternating current at 50 Hz. Standards also exist in various countries, etc. For equipment that will be connected to the power supply or mains, the harmonics of the power supply current that will be produced by such equipment have to meet requirements, for example, the standard IEC 61000-3-2.
To insure that the power supply signal meets these harmonics requirements, EMC (electro magnetic compatibility) measurements are taken. The equipment used to make these measurements has to be calibrated, for example, every year. However, methods for determining whether the test equipment is properly calibrated and produces the correct results (referred to as determining the plausibility) are difficult to perform, and thus, not performed very often.